As is known in the art, aircraft are equipped with transponders to help air traffic control (ATC) radar systems identify and track civilian aircraft. So-called Mode A or C transponders (Mode A/C transponders) receive Mode A or C interrogation signals transmitted at a frequency of 1030 MHz from secondary surveillance radars (SSRs), such as ASR-9 radars. The Mode A/C transponders respond to the interrogation signals at a frequency of 1090 MHz with the aircraft identity (beacon code) and altitude respectively. The SSR couples this information with the antenna azimuth, and derived range and provides a position report to a local ATC system.
Another type of transponder is a so-called Mode S transponder. Mode S transponders operate on a similar principle to Mode A/C transponders but include enhancements to minimize interference caused by responses to interrogations from adjacent secondary radars. A protocol is established to select and address interrogations to specific aircraft.
As is also known, Automatic Dependent Surveillance Broadcast (ADS-B) is a capability added to Mode S transponders. As described by the FAA (Federal Aeronautics Administration) ADS-B is a technology which uses signals from Global Positioning Satellites (GPS), instead of radar data, to determine aircraft position.
ADS-B systems broadcast aircraft position, altitude, velocity and identity on a period of 400-600 msec for position, and 5000 msec for identify (e.g. call sign or flight name). All messages include a unique Mode S address, and the identity includes the aircraft call sign used on the flight plan (e.g. FDX1964).
These messages are broadcast as Mode S “responses” at a frequency of 1090 MHz and are received by ADS-B ground receivers where the position, altitude, velocity and identity is extracted and provided to an ATC automation system. These signals are also received by airborne ADS-B receivers where the data is provided to the cockpit display.